Wavelength division multiplexing (WDM) has been explored as an approach for increasing the capacity of fiber optic networks. In a WDM system, plural optical signals or channels are combined onto a single optical fiber with each channel being assigned a particular wavelength. Such systems typically include a demultiplexer for separating and supplying the channels to corresponding receivers for further processing or retransmission of data carried by the channels.
Optical signals or channels in a WDM system are frequently transmitted over silica based optical fibers, which typically have relatively low loss at wavelengths within a range of 1525 nm to 1580 nm. WDM optical signal channels at wavelengths within this low loss “window” can be transmitted over distances of approximately 50-100 km and remain detectable by conventional receiver circuits. For distances beyond 100 km, however, optical amplifiers are required to compensate for optical fiber loss.
Optical amplifiers have been developed which include an optical fiber doped with erbium. The erbium-doped fiber is “pumped” with light at a selected wavelength, e.g., 980 nm, to provide amplification or gain at wavelengths within the low loss window of the optical fiber. However, erbium doped fiber amplifiers do not uniformly amplify light within the spectral region of 1525 to 1580 nm. For example, an optical channel at a wavelength of 1540 nm, for example, is typically amplified 4 dB more than an optical channel at a wavelength of 1555 nm. While such a large variation in gain can be tolerated in a system with only one optical amplifier, it cannot be tolerated for a system with plural optical amplifiers or numerous, narrowly spaced optical channels. In which case, much of the pump power supplies energy for amplifying light at the high gain wavelengths rather than amplifying the low gain wavelengths. As a result, low gain wavelengths suffer excessive noise accumulation after propagating through several amplifiers.
Accordingly, optical filters have been incorporated into so-called “gain flattened” amplifiers to provide a substantially uniform amplifier gain spectrum. Moreover, a variable optical attenuator can be provided mid-stage in an optical amplifier to provide gain flatness over a wide range of optical input power levels, as discussed further in U.S. Pat. No. 6,057,959, incorporated by reference herein.
Gain flattened amplifiers have often been deployed in systems in which each WDM signal carries data at the same rate. In many applications, however, system requirements may dictate multiple WDM signals carrying both high and low data rate channels. The high data rate channels comprise relatively short pulses having higher intensity than the longer duration, lower intensity pulses that make up the lower rate channels. Since conventional optical amplifiers are typically configured to provide uniform gain for a given signal strength, such amplifiers may impart insufficient gain to the high data rate channels. In which case, a non-uniform spectrum is desired whereby each of the high data rate channels has a first gain, while each of the lower data rate channels has a second, preferably lower gain. Conventional gain flattened optical amplifiers are configured to have a uniform gain for WDM channels at a given intensity level, such as those having the same data rate, and therefore cannot provide the requisite gain spectrum for WDM systems optical signals at different data rates.